Technical Field
Disclosed herein are fan blades for turbofan gas turbine engines. In one example, a disclosed fan blade may include a metallic body having one or more hollow cavities for weight reduction and a segmented cover disposed on a the suction side of the fan blade and that encloses the hollow cavities. The disclosed segmented cover reduces the risk of damage to downstream engine components in the event of damage to the fan blade that causes partial or complete dislodgement of the cover from the fan blade.
Description of the Related Art
A geared turbofan engine is a type of turbofan airplane engine, similar to a turbojet. A geared turbofan engines, also known as a type of a gas turbine engine, may include a geared, ducted fan and a smaller diameter gas turbine engine mounted behind the fan that powers the fan. Part of the airstream passes through a core of the engine, which includes low and high-pressure compressors, a combustion chamber and high and low pressure turbines. The high and low-pressure turbines are disposed downstream of the combustor between the combustor and an exhaust. In contrast, the low and high compressors are disposed upstream of the combustor and between the combustor and fan. The high and low-pressure turbines drive the high and low-pressure compressors respectively and the fan.
Weight reduction of gas turbine engines used for aircraft results in fuel savings. One known means for reducing the weight of a gas turbine engine is to include hollow cavities in some of the components that do not need to be solid metal to meet structural requirements. One such component is a fan blade, also known as a type of airfoil. Some fan blades include a titanium or aluminum body with recesses or cavities disposed in the non-flow path convex side of the fan blade, also known as the suction side of the fan blade. The opposite side of the fan blade is the concave or pressure side. The cavities may be covered by a composite cover, typically made from fibers and resin, and the fan blade is the then covered with a damage resistant coating that is typically non-structural and inert.
During engine operation, a fan blade or a fragment thereof may separate from the remainder of the fan (a so-called “fan blade-off” or “fan blade-out” event (FBO)). An FBO event may occur as a result of a foreign object striking one or more of the fan blades, or during a FOD (foreign object damage) event. One portion of a fan blade that is vulnerable to separation from the fan blade body is the composite cover for the cavities. Partial or complete separation of the cover from one or more fan blades can cause damage to a downstream component of the engine. The damage caused by the separated cover or partial separated cover may depend on numerous factors, including the size and mass of the separated cover or fragment, the design of the downstream engine components, etc. Further, partial or complete separation of a cover may displace the center of gravity (center of mass) of the entire fan assembly from its central axis. At least initially, bearings may constrain the fan radially so that it continues to rotate about its central axis rather than about the displaced center of gravity. However, if the bearings fail, rotation of the fan about the displaced center of gravity may result in forces that may also damage other downstream engine components.
If an FBO event severely damages the engine, the engine may cease normal operation, shut down or lock, and consequently produce no further power. However, despite the engine shut down, it is undesirable to stop rotation of the fan. If rotation of the fan stops, the engine becomes an extreme source of aerodynamic drag for the aircraft. Such drag would be particularly significant in a twin-engine aircraft, with one engine mounted to each wing nacelle. This is a common construction for many passenger aircraft. Thus, in a twin-engine aircraft, the combination of drag from the shut down engine and thrust from the remaining engine would produce an excessive yawing moment not easily overcome by the aircraft rudder.
To overcome this problem, the fan of a shut down or locked gas turbine engine may be designed continue to spin at above-idle speed as air is forced through the fan due to forward aircraft motion. This unpowered fan rotation is called “windmilling.” Even a fan of a shut-down engine on the ground may be designed to windmill. A windmilling engine has less aerodynamic drag than does a completely stopped or locked engine. To remain windmilling, the engine must resist damage to the turbine, bearings, etc.
The engine must also be configured to avoid catastrophic damage, which may be caused by fan blade failure, and which might permit fan blade portions to enter the high-pressure turbine. If a part or debris enters the high-pressure turbine, for example, centrifugal forces may cause the parts or debris to puncture one or both of the nacelles, the fuselage or allow the engine to detach from the aircraft or damage the wing.
Hence, there is a need for an improved fan blade design that is lightweight, includes hollow cavities that are covered, but that includes a means for covering the hollow cavities that will provide better resistance to damage in the event of any FBO-causing event.